This application claims priority to Japanese Patent Applications Nos. Hei 9(1997)-4491 and Hei 9(1997)-144161, each disclosure of which is incorporated by reference in its entity.
1. Field of the Invention
The present invention relates to a microorganism holding carrier for a fluidized bed for use in a biochemical reactor used in a sewage treatment facility which treats sewage by utilizing microorganisms.
2. Description of the Related Art
A fluidized-bed biochemical reactor used in a sewage treatment facility that utilizes biochemical reactions clarifies sewage by fluidizing carriers. These carries are thrown into a treatment tank by an air-seasoning operation to hold sewage-treating microorganisms, i.e., activated sludge, on the carriers.
Generally required features of the microorganism holding carrier for use in a fluidized bed in this kind of sewage treatment facility are that the carrier is: (1) superior in fluidity so as to quickly become familiar with water to uniformly fluidize in the treatment tank; (2) porous so as to be easily held by microorganisms; and (3) strong enough to endure a fluidization, i.e., superior in durability such as an abrasion resistance.
Under these circumstances, diatomaceous earth, a porous inorganic substance, is conventionally used as a microorganism holding carrier. However, because such a carrier is about 2.0 g/cm.sup.3 in apparent density, which is much heavier than water, a strong mixing power is required to fluidize them, which causes an increase of carrier fluidizing energy, causing an increase of running cost. Further, because such a diatomaceous earth as a carrier is easily worn when fluidized, it is hard to maintain the initial configuration for a long period, thus lacking durability.
Recently, a plastic carrier made of a synthetic resin which is similar to water in density, thereby requiring a decreased fluidizing energy, and which is superior in durability, has been developed and has rapidly become popular.
In manufacturing this kind of synthetic resin carrier, generally adapted is a method utilizing a porous-making technique such as a dropping forming method or a sintering method, or a method utilizing an extruding foaming method.
However, in manufacturing a microorganism holding carrier by utilizing a porous-making technique such as a dropping forming method or a sintering method, it is difficult to attain high productivity and is also difficult to use cheaper materials due to the limited synthetic resin materials. This increases the manufacturing cost.
In case of utilizing an extruding foaming method, though the productive efficiency may be improved, the extruding foaming method in manufacturing the carrier has not yet been fully established. As a result, synthetic resin carriers made by a conventional extruding foaming method are of uneven quality, causing problems such as bad water-familiarity or bad fluidity.
On the other hand, in recent years, it has been suspected and is now a social problem that a waste disposal of spent plastic goods or articles may exert a harmful influence on the environment. This problem is not an exception in the biochemical reactor technical field. Thus, it is now strongly required to use a synthetic resin carrier which exerts less harmful influence on the environment.